The objectives of the proposed research are as follows: 1) To assess the contribution of extracellular factors, including exotoxin and proteolytic enzymes, produced by Pseudomonas aeruginosa multiplying in vitro, to infection. 2) To study the initial binding and subsequent internalization of pseudomonas exotoxin by mammalian cells in culture; to determine if uptake is mediated by specific receptors and to compare the action of toxin on sensitive and resistant mammalian cells. 3) To characterize factors in burned skin which facilitate rapid multiplication of P. aeruginosa. All of these objectives are related to characterizing the infection of burned tissues by P. aeruginosa. Data obtained to date suggest that both exotoxin and proteolytic enzymes are important to the expression of virulence in a burned mouse model. The contribution of these factors both in vivo and in vitro will be examined. This will include studying the role of proteolytic enzymes in the activation of toxin and on the dissemination of toxin and bacteria from the burn site. It will also involve examining their effects on the host defense system. Uptake of labeled toxin by sensitive and resistant cells will be monitored.